JPH0314214B2 - - Google Patents

Description

Detailed Description of the Invention [Field of the Invention] The present invention relates to a magnetic field coil that generates at least an approximately uniform magnetic field in the internal space of an air-centered cylindrical coil, and particularly to a nuclear magnetic resonance computer tomography apparatus ( This invention relates to a magnetic field coil for NMR-CT (hereinafter referred to as NMR-CT).

[Prior art and its problems]

The uniform magnetic field coil used for NMR-CT is
In order to store the human body as a test subject inside the coil,
Consisting of a cylindrical or multiple ring-shaped coil array with an inner diameter of nearly 1 m, in order to obtain a tomographic image sufficient to determine the human body being examined, the strength of the magnetic field in the inner space of the coil where the human body is housed is required. Uniformity on the order of tens of parts per million is required. In order to satisfy the strength and uniformity of this magnetic field, a method is known in which a uniform magnetic field coil is constructed by combining multiple coaxially arranged ring-shaped main coils and selecting the arrangement and current value that provide the best uniformity of the magnetic field. There is.

1 to 3 are cross-sectional views showing a coil arrangement of a uniform magnetic field coil consisting of a plurality of ring-shaped main field coils. In the case of FIG. 1, the uniform magnetic field coil consists of a pair of ring-shaped main coils 11 and 12, coaxial to the (virtual) rotation axis 1 and symmetrical to the symmetry plane 2 perpendicular to the rotation axis 1. are arranged in parallel. In the case of FIG. 2, the uniform magnetic field coil consists of four ring-shaped main coils coaxial with the rotation axis 1, and consists of coils 21 and 22 arranged at symmetrical positions with respect to the plane of symmetry 2. It is composed of two pairs of coils, one pair of coils and another pair of coils consisting of coils 23 and 24, and the arrangement, size, current value, etc. of each coil are adjusted so that the uniformity of the magnetic field is the best. By choosing
It is constructed to improve the uniformity of the magnetic field compared to the magnetic field coil constructed from a pair of coils in FIG. Figure 3 shows a uniform magnetic field coil consisting of two pairs of ring-shaped main coils 31, 32 and 33, 34, similar to Figure 2. ) to improve the uniformity of the magnetic field strength.

FIG. 4 is a magnetic flux distribution diagram obtained as an example of the magnetic velocity distribution diagram of a uniform magnetic field coil as a coaxial cylindrical coordinate system with rotation axis 1 for the coil arrangement of FIG. This figure shows the magnetic flux distribution in the upper right part including the upper right part. In the figure, 100
are magnetic flux lines generated by the current flowing through the four ring-shaped main coils 31 to 34;
4, and the magnetic flux lines of the part passing through the inner diameter side of the main coils 31 and 32 are parallel to the rotation axis 1 except for the part near the coils, and the strength of the magnetic field in the inner space of the coils is It shows that is uniform. On the other hand, the magnetic flux lines leaking to the outside of the coils 31 and 32 are distributed over a wide range in the axial and radial directions outside the coils. Here, the magnetic field leaking to the outside of the magnetic field coil will be referred to as a leakage magnetic field. Assuming that a magnetic field coil that generates a leakage magnetic field that spreads as described above is installed in a room of a certain size, if there is a ferromagnetic substance in the room, the leakage magnetic field will be affected by it and the distribution of the magnetic field will change. As a result, a problem arises in that the strength of the magnetic field within the uniform magnetic field coil and its uniformity are considerably affected.

In particular, in Japan, it is generally difficult to prepare a room with a radius of, for example, 5 mm in order to install an NMA-CT, and leakage magnetic field distribution due to steel materials such as reinforcing bars and steel frames of buildings is generally difficult. The reality is that it is difficult to prevent the distortion of

In order to create a uniform magnetic field in the inner space of the coil with an accuracy of several tens of millionths,
Each coil is generally configured to be supported such that its relative position relative to each other can be finely adjusted. Optimum coil positioning is performed to achieve the above-mentioned accuracy under constrained ambient conditions and often requires inspection and modification to maintain accuracy. In some cases, in order to maintain accuracy, consideration must be taken to prevent moving objects such as cars from approaching.

Therefore, in order to reduce the range in which the leakage magnetic field is distributed and the amount of its magnetic flux, a cylindrical body made of ferromagnetic material is provided outside the coil, and the leakage magnetic field is configured to circulate inside the coil through this cylindrical body. There is a known method of preventing leakage magnetic fields from widely distributing indoors. However, the thickness of the cylindrical body needs to be several tens of millimeters, which increases the weight of the entire uniform magnetic field coil device, which poses problems such as hindering the strength of the floor at the installation site and hindering transportation and loading. Since the magnetic properties of a cylindrical body made of a magnetic material are nonlinear, there is a problem in that precise numerical calculation of the magnetic field is difficult when designing a magnetic field coil with good uniformity.

[Purpose of the invention]

The present invention was made in view of the above-mentioned situation, and
It is an object of the present invention to provide a uniform magnetic field coil that can prevent the spread of leakage magnetic fields without providing a cylindrical body made of ferromagnetic material.

[Key points of the invention]

The coil of the present invention has one or more pairs of ring-shaped main coils coaxially arranged on the outside of a magnetic field coil consisting of one or more pairs of ring-shaped main coils, and having the same magnetic efficiency as the main coil and opposite direction of generated magnetic flux. A shield coil is provided to suppress the spread of the leakage magnetic field to the outside of the shield coil by canceling the leakage magnetic field, and by increasing the diameter of the shield coil, the weight and current of the shield coil are reduced. This suppresses increases in weight and power consumption due to the provision of a coil.

Here, the magnetic efficiency is the sum of the products of the magnetically effective area and ampere turns of each individual coil, and is determined by the following equation. Now, if we assume that there is a circular wire ring with a sufficiently small cross section and made up of a single conductor, the magnetic efficiency of this wire ring can be expressed as the product of the area of the circle surrounded by this conductor and the current flowing through the conductor. In other words, m=si...(1) where, m: Magnetic efficiency of this wire s: Area of the circle surrounded by the conductor of this wire (radius is r
Then, s = πr ² ) i: Current flowing in the conductor If the magnetic efficiency of the main coil is M ₁ , then M ₁ can be calculated by integrating the above equation (1) for each conductor of each wire that makes up this main coil. good. That is, M ₁ = μ ₀ { _C1 〓 ^N=1 _NN 〓 ^j=1 πr ² _Nj }I ₁ ...(2) Here, μ ₀ ; Magnetic permeability in vacuum (4π×10 ^-7 ) C ₁ ; Number of wires of the first coil N _N ; Number of turns of the Nth wire rN _j ; Radial position of the conductor I ₁ ; Current value of the conductor of the first coil Also, the magnetic efficiency M ₂ of the shield coil is indexed by the above formula. Just change 1 to 2. However, since the direction of the current is opposite, if the magnetic efficiency of the main coil is positive, that of the shield coil is negative. The relationship between the above two coil magnetic efficiencies is expressed using M ₁ and M ₂ as follows.

M ₁ +M ² =0 (3) Furthermore, the magnetic flux density B _1r of the magnetic field generated by the main coil at a position sufficiently distant from these coils is approximately expressed by the following equation.

B _1r =M ₁ /2π{k ₃ 1/r ₃ +k ₅ a ² _1e r ⁵ +………}……(4
) Here, r: Distance from the center of the coil a _1e ; Equivalent radius of the main coil k ₃ , k ₅ ; Coefficient of each order term in the series expansion (not related to the equivalent radius a _1e ) As can be seen from this formula The strength of the leakage magnetic field generated by a single coil to the outside of the coil mainly decreases in inverse proportion to the cube of the distance from the center of the coil. In the case of an NMR-CT magnet with a magnetic flux density of around 1T, it is not possible to expect attenuation to the extent that this leakage magnetic field does not affect the neighboring room several meters away from the NMR-CT device or NMR-CT devices other than the magnet. It is. From this point of view, it becomes necessary to provide another coil as a shield coil.

The magnetic flux density B _2r of the shield coil is expressed by the following formula in the same way as the main coil.

B _2r = M ₂ {k ₃ 1/r ³ +k ₅ a _2e /r ⁵ +......} ...(5) The magnetic fields generated by these two coils are as described above.
It is expressed as the sum of equations (4) and (5), and becomes the following equation.

_Br ＝ _k3M1 ＋ _M2 ／ _r3 ＋ _k5a2 ／ _1e ^M1 ＋ ^a2 ／ _{2e M2} ^／r5 _＋ …
… … (6) By substituting formula (3) into this formula, it can be rearranged as follows.

Br＝k ₅ (a ² / _1e −a ² / _2e )M ₁ /r ⁵ +… (7) In other words, if the magnetic efficiencies of the two coils, the main coil and the shield coil, are matched and their directions are reversed, By doing this, the strength of the leakage magnetic field is such that the component that is inversely proportional to the cube of the distance from the center of the coil becomes zero, and the component that decreases inversely proportional to the fifth power becomes the main component.
As the distance from the coil center increases, the leakage magnetic field decreases rapidly. For example, if the radius of the main coil is 0.5 m and the radius of the shield coil is 7.7 m, and the magnetic flux density of the uniform magnetic field at the center of the coil generated by these two coils is 1 T, then the leakage magnetic field at a position 3 m away from the center of the coil is The magnetic flux density of is approximately
1.3G, which is a sufficient reduction effect. By the way, when a uniform magnetic field of 1T is generated at the center of the coil using only the main coil, the magnetic flux density of the leakage magnetic field at a distance of 3m from the center of the coil is approximately 50G, which is much higher than the allowable value of 5G for the strength of the leakage magnetic field. becomes a large value.

In this way, by matching the magnetic efficiency values of the two coils, the strength of the leakage magnetic field can be canceled out, but this magnetic efficiency can be easily calculated by setting the number of turns and dimensions of the wire that makes up the coil. Therefore, in the process of designing a uniform magnetic field coil, it is possible to proceed with calculations while always satisfying the condition that the magnetic efficiencies match, making it possible to perform efficient design calculations.

[Embodiments of the invention]

The present invention will be explained below based on examples.

FIG. 5 is a coil arrangement diagram showing the most basic embodiment of the present invention, and shows an example of application to a magnetic field coil consisting of a pair of ring-shaped main coils shown in FIG. In the figure, 41 and 42 are a pair of shield coils, which are arranged coaxially with the main coils 11 and 12 and symmetrically with respect to the plane of symmetry 2.
is itself constructed to satisfy the best magnetic field homogeneity. In addition, the shield coil and the main coil are formed so that the direction of the generated magnetic flux is opposite to each other, and the magnetic efficiency of the pair of shield coils (the sum of the products of the magnetically effective area and ampere turns of each individual coil) is calculated as shown in the following equation. ) is configured to be equal to that of the main coil.

In Figure 5, the diameter of the shield coil is chosen to be twice the diameter of the main coil, so the ampere turns of the shield coil is 1/4 of the ampere turns of the main coil according to equation (2) above. It turns out.
Furthermore, the strength of the uniform magnetic field near the rotating shaft 1 is inversely proportional to the diameter of the coil, so the strength of the uniform magnetic field in the opposite direction created by the shield coil is 1/8 of the strength of the uniform magnetic field created by the main coil. . Therefore, the strength of the uniform magnetic field is reduced to 7/8 by providing the shield coil.
decreases to In order to keep the strength of the uniform magnetic field the same as in the conventional structure without a shield coil, the ampere turns of the main coils 11 and 12 may be increased to 8/7 times in advance.

FIG. 6 is a magnetic flux distribution diagram of the uniform magnetic field coil of FIG. 5. In the figure, the magnetic flux lines 100 surrounding the main coils 11 and 12 are almost parallel to the rotation axis 1 in the part where they pass inside the main coil 11 (lower side in the figure), and the strength of the magnetic field in the space inside the main coil is This shows that it is almost uniform. Furthermore, the distance between the magnetic flux lines 101 and 102 that pass outside the shield coil 41 is greatly increased at a position slightly away from the shield coil 41, and the strength of the leakage magnetic field outside the shield coil is significantly reduced. It is shown that. In this situation, by comparing the distribution of magnetic flux lines in FIG. 4 with the distribution of magnetic flux lines in FIG. 6, the effect of reducing the leakage magnetic field by the shield coil can be clearly understood.

FIG. 7 is a coil arrangement diagram of a uniform magnetic field coil showing another embodiment of the present invention, and shows an example of application to a magnetic field coil using the main coil arrangement of FIG. 3.
In the case of the figure, a pair of shield coils 51 and 52 provided outside the two pairs of main coils 31, 32 and 33, 34 have equal ampere turns and are arranged symmetrically with respect to the plane of symmetry 2. , the other pair of shield coils 53 and 54 also have the same ampere turns and are arranged at symmetrical positions with respect to the plane of symmetry 2. In addition, a pair of main coils 31 and 32
The ampere turns of the other pair of main coils 33, 3
It is 1/2 of that of 4, and 2 pairs of shield coils 5
1, 52 and 53, 54 are both configured to have the same ampere turns. However, any ratio can be selected as long as the main coil group and the shield coil group each satisfy the condition that a uniform magnetic field is formed. Further, the ampere-turn ratio between the main coil and the shield coil is determined so as to satisfy the above-mentioned conditional expression.

FIG. 8 is a magnetic flux distribution diagram in the coil arrangement of FIG. 7. In the figure, main coils 31 and 33
The magnetic flux lines 100 surrounding the coils 31 and 3
3 (lower side in the figure), it is almost parallel to the rotation axis 1 as in the case of Figure 4, indicating that the strength of the magnetic field in the main coil internal space is uniform. There is. Also, the shield coil 51
and magnetic flux lines passing outside of 53, for example 103,
104 has a large gap between the outside of the shield coil and the inside magnetic flux line, and as is clear from the comparison with Fig. 4, the spread and strength of the leakage magnetic field are significantly reduced. I understand.

Note that although the embodiment corresponding to the main coil arrangement in FIG. 2 has been omitted, it can be easily inferred from the above-mentioned embodiment that the present invention can be applied to any combination of the number and arrangement of main coils. be able to.

Also, by increasing the ratio of the radius and ampere turns of the main coil to the shield coil, the increase in current in the main coil can be reduced, but on the other hand, the diameter of the entire coil becomes larger, and the ratio Since the increase in the ampere turns of the main coil will increase if the value is made smaller, it is preferable to determine the optimum conditions by taking various performances required of the uniform magnetic field coil into consideration.

〔Effect of the invention〕

As described above, the present invention includes a plurality of ring-shaped shield coils whose sum of magnetic efficiencies is equal to that of the main coils and whose direction of generated magnetic flux is opposite to that of the main coils, on the outside of a uniform magnetic field coil consisting of a plurality of ring-shaped main coils. It was configured to provide. As a result, it is possible to provide a uniform magnetic field coil in which the spread and strength of the leakage magnetic field that spreads outside the uniform magnetic field coil is significantly reduced compared to conventional uniform magnetic field coils that do not have a magnetic circuit. Conventional problems such as increased inhomogeneity of the uniform magnetic field caused by the linkage of leakage magnetic fields to ferromagnetic materials existing in This can contribute to preventing disturbances in tomographic images of the human body produced by NMR-CT.

In addition, shield coils can be made much lighter than cylindrical bodies made of ferromagnetic material, so there is no need to transport them to the installation site or pay special attention to floor strength, and NMR can be easily installed inside existing buildings. −
CT can be provided.

Furthermore, when fine-tuning and correcting disturbances in the uniform magnetic field, the position of the shield coil, which has a fraction of the ampere-turns compared to the main coil and is therefore lightweight, can be supported in an adjustable manner. The coil support structure can be simplified compared to a method that supports the main coil in an adjustable manner, and the amount of magnetic flux generated by the shield coil that accounts for the strength of the uniform magnetic field is small, compared to adjusting the position of the main coil. This has the advantage that it becomes possible to more finely correct disturbances in the uniform magnetic field.

Furthermore, since the main coil does not include a part with nonlinear magnetic properties such as a cylindrical body made of ferromagnetic material, highly accurate numerical calculation of the magnetic field is possible when designing a uniform magnetic field coil. It has the advantage of being easy to do.

[Brief explanation of drawings]

Figures 1 to 3 are arrangement diagrams of the main coil of a known uniform magnetic field coil, Figure 4 is a conventional magnetic flux distribution diagram in the main coil arrangement of Figure 3, and Figure 5 is a coil showing an embodiment of the present invention. 6 is a magnetic flux distribution diagram in the embodiment of FIG. 5, FIG. 7 is a coil layout diagram showing a different embodiment of the present invention, and FIG. 8 is a magnetic flux distribution diagram in the embodiment of FIG. 7. . 1...Rotation axis, 2...Symmetry plane, 11, 12...
A pair of main coils, 21, 22, 23, 24... Two pairs of main coils with different diameters, 31, 32, 33, 3
4...Two pairs of main coils with different ampere turns,
41, 42...Pair of shield coils, 51, 5
2, 53, 54...Two pairs of shield coils with different diameters, 100...Magnetic flux lines surrounding the main coil, 1
01, 103, 104...Leakage magnetic flux lines.

Claims (1)

[Claims] 1. A magnetic field coil that includes at least a pair of ring-shaped main field coils that are coaxial with each other and symmetrical and parallel to a plane of symmetry perpendicular to the axis, and that generates a uniform magnetic field in the internal space of the main field coil, at least a pair of ring-shaped shield coils having an inner diameter larger than the outer diameter of the main field coil, coaxial with the main field coil, and symmetrically arranged with respect to the plane of symmetry; A uniform magnetic field coil characterized in that the sum of the magnetic efficiencies of is equal to that of the main field coil, and the direction of the generated magnetic flux is opposite to that of the main field coil.